The present invention relates to a copier, printer, facsimile machine or similar image forming apparatus for forming an image by modulating a light beam signal by an image signal, deflecting the modulated light beam signal by a polygonal mirror to irradiate a photoconductive element for thereby electrostatically forming a latent image, and developing the latent image.
With an image forming apparatus of the type described, there is an increasing demand for high-density multi-tone images. Banding of an image in the subscanning direction has been discussed as a critical phenomenon which degrades the quality of images. Banding is ascribable to the irregularities in the configuration of a polygonal mirror, i.e., the irregular angles of the individual mirrors of the polygonal mirror and the fluctuation of the rotation of a photoconductive element. The irregular angles of the individual mirrors cause a beam spot formed by a laser beam on a photoconductive element to vary in position. On the other hand, the fluctuation of the rotation of the photoconductive element causes the pitch in the subscanning direction to vary.
With respect to the banding ascribable to the irregularities in the configuration of the polygonal mirror, an approach for eliminating this problem is disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 91211/1981. The disclosed approach is such that a beam irradiates a mirror surface different from a mirror surface which should be irradiated by a data beam, thereby producing a signal representative of an inclination of the mirror surface beforehand. When the mirror surface of interest has reached a predetermined position where it is to be irradiated by a data beam, the incidence angle of the data beam to the polygonal mirror is controlled on the basis of the inclination signal.
As for the elimination of the banding due to the fluctuation in the rotation of the photoconductive element, some elaborated implementations are taught in Japanese Utility Model Laid-Open Publication No. 193518/1985 and Japanese Patent Laid-Open Publication Nos. 7262/1987, 65275/1986, and 110115/1986. Specifically, Utility Model Laid-Open Publication No. 193518/1985 uses an actuator for mechanically displacing a laser or a collimator lens placed in parallel to the axis of rotation of a polygonal mirror and in synchronism with the rotation of the mirror. Patent Laid-Open Publication No. 7262/1987 senses the angular positions of a photoconductive element and thereby sets the timings for beginning and ending imagewise exposure. Patent Laid-Open Publication No. 65275/1986 discloses a system which optically writes an image on a photoconductive element by an optical writing element in synchronism with the transport of the photoconductive element. Further, Patent Laid-Open Publication No. 110115/1986 proposes a system for rotating, when a laser beam deflected by a polygonal mirror is detected, a photoconductive element by a given angle which is determined by the interval between successive scanning lines.
A problem with the approach of Patent Laid-Open Publication No. 91211/1981 is that the construction is complicated and, moreover, it does not give any considerations to the fluctuation of the rotation of a photoconductive element. The implementation of Utility Model Publication No. 193518/1985 which relies on mechanical displacements is not satisfactory when it comes to a rapid response to speed ripples or other similar problems. The systems of Patent Laid-Open Publication Nos. 7262/1987 and 65275/1986 are not limited to LD arrays and LCD arrays but also impracticable without resorting to complicated circuitry. The scheme of Patent Laid-Open Publication No. 110115/1986 uses gears and a timing belt for controllably driving a photoconductive element and, therefore, it cannot eliminate the fluctuation of the rotation of the photoconductive element.